A measuring axis of an interferometer system is understood to mean an axis along which the position or the displacement in a given direction (X or Y) of a given point of an object is measured. This measuring axis does not need to coincide with the chief ray of the measuring beam which is used for the measurement. If the measuring beam is sent through the system twice and reflected by the object twice at substantially the same point, the measuring axis is situated between the chief ray of the measuring beam on the first passage and the chief ray of this beam on the second passage.
An interferometer system for use in a lithographic apparatus is known from, inter alia, U.S. Pat. No. 5,404,202. The lithographic apparatus is used for repetitive and reduced imaging of a mask pattern, for example the pattern of an integrated circuit (IC) on a substrate provided with a radiation-sensitive layer. Between two successive images of the mask pattern on the same substrate, this substrate and the mask are moved with respect to each other, for example parallel to the X or Y direction of an XYZ system of coordinates, while the substrate plane and the mask plane are parallel to the XY plane so as to image the mask pattern consecutively on all substrate fields, or IC areas, of the substrate.
In the manufacture of integrated circuits, the lithographic apparatus is used in combination with masking and diffusion techniques. A first mask pattern is imaged in a large number, for example, dozens of fields of a substrate. The substrate is then removed from the apparatus so as to subject it to the desired physical and/or chemical process steps. Subsequently, the substrate is introduced into the same or another similar apparatus so as to form an image of a second mask pattern in the different substrate fields, and so forth. It should then be ensured that, for the images of mask patterns on a substrate field, this field and the mask pattern should be aligned very accurately with respect to each other. To this end, a lithographic apparatus not only comprises an alignment system but also an interferometer system. In a stepping apparatus, the interferometer system is used to measure the movements and the positions of the substrate and the separate substrate fields accurately. In a step-and-scanning apparatus, not only this substrate interferometer system but also a mask interferometer system is used to check whether, during illumination of a substrate field, the substrate and the mask move synchronously with respect to the projection beam and a projection system by means of which the mask pattern is imaged on a substrate field.
Since it is desirable to provide an IC with an increasingly larger number of electronic components, which means that the details of these components should become smaller, more stringent requirements must be imposed, not only on the resolving power and the imaging quality of the projection system but also on the accuracy with which the positions of the substrate fields can be measured and thus checked. This means that the interferometer system must also become more accurate. Then, turbulences and other disturbances, particularly in the medium in which the measuring beams propagate, will start playing an important role. These turbulences and disturbances cause variations of the refractive index of the medium, which variations are interpreted as displacements by the interferometer system.
U.S. Pat. No. 5,404,222 describes an interferometer system in which the effects of the disturbances can be measured so that the position measurements can be corrected therefor. This interferometer system comprises a first laser source in the form of a HeNe laser which supplies a beam at a wavelength of 633 nm. This beam is split into a measuring beam and a reference beam which are sent to the measuring mirror and the reference mirror, respectively, and with which the position of the measuring mirror is measured in known manner. The known interferometer system further comprises a second laser source which supplies two test beams at considerably different wavelengths. These test beams both traverse the measuring path to the measuring mirror and, after reflection by this mirror, they reach a special detection system. Use is made of the fact that, for a beam, the refractive index of the medium traversed by this beam is dependent on the wavelength of this beam. When said disturbances occur, the refractive index variations for the two test beams are different, resulting in a phase difference between the two test beams. By measuring this phase difference, the effect of the disturbances on the measuring beam traversing the same medium can be measured. Since use is made of the dispersion of the medium, the wavelengths of the test beams should be considerably different for an accurate measurement of the effects. In the interferometer system described in U.S. Pat. No. 5,404,222, wavelengths of 532 nm and 266 nm have been chosen. The first test beam with a wavelength of 532 nm is supplied by a laser, and the second test beam with a wavelength of 266 nm is obtained from the first test beam by a frequency-doubling element which comprises a non-linear material and converts a part of the radiation with a wavelength of 532 nm into radiation with a wavelength of 266 nm which constitutes the second test beam. In practice, the problem occurs that the interferometer system comprises a plurality of elements which are wavelength-dependent by nature. For example, the conventional interferometer systems comprise a polarization-sensitive beam splitter and a .lambda./4 plate in which .lambda. is the wavelength used, by means of which it can be ensured that substantially no radiation loss occurs when splitting the laser beam into a measuring beam and a reference beam and when combining these beams again. It is very difficult, if not impossible, to manufacture such elements which are suitable for both the wavelength of the measuring beam and for the two wavelengths of the test beams. The same applies to multilayer reflection structures and multilayer antireflection structures which are provided on components of the interferometer system.